Pistons with Oil Cut-outs

Hi All,

Just wanted to have people's view on using pistons that have the oil cut outs in them. I realise the ones that have a huge slot extending around nearly around the circumference near the oil ring are bad but what about the later ones, think they were used in the Combat. The link below shows them well.

http://www.norvilmotorcycle.co.uk/techtalk34.htm

Its in a 71 750. Im not trying to avoid getting new pistons which in turn will most likely require a rebalance and a bore/clearance check/rebore (havnt measured yet) but just after thoughts.

Thanks all.
Cant wait for the replies

I know the reason the 'comma' shaped oil slot pistons were eliminated as a stress riser danger that can part top of piston form rod blow up surprise not even pressing luck enjoying Commando torque/rpm response. My recomendation is save/display the slotted/comma weak pistons on shelf and get newer versions w/o oil holes. Not hobot hearsay, but facts of longer life riding care free.

Thanks Hobot

Ugh I meant new piston with small round oil holes, not w/o any holes. In their day comma hole pistons did ok but Alu has a fatigue factor in light built pistons so over some decades = bad juju. My 2nd Combat was shop stored unattended a decade or more but was intact and got runing nice to just commute around timidly to have it seize on level road, 3 wks later just sightseeing at 50's mph. Found parted comma piston aggravated by Alu gasket that had mooshed into bore enough to rub on pistons at TDC jerk down. Messed up rod and parts nicked lifters and cam and punched cracks TS crank case. I only want to be a pilot but forced to learn quirks of Commando recovery. Also learned essentially any short cut in exams and recovery work arounds usually don't work well long. A few seasons later working nice again had another seize blow up after 3wks timid use d/t a corrosion crack in rod cap bolt finally crossing the last straw/metal grain boundary. I had new nuts but skipped new bolts, ugh. Got Trixie going again after a year or so but would smoke if used with spirit to finally break down and clear case oil passage in 2015 but too many other crisis, so still awaiting slack to get cylinder processed for new pistons.

The price of a set of rings compared to the price of a new piston kit makes the pistons very cheap

In 1975 I cracked one of the slotted pistons after running massively over advanced timing. At the time replacement pistons were also slotted most of the way round. I was ignorant of anything else in those days. These type lived in my bike for about 60,000 miles with no problem. Admittedly I didn't spend hours on end at massive RPMs but they did get a caning now and then. When I did go for a rebore, the old pistons still looked in excellent condition but knowing now what they are, I don't suppose they are fit for anything other than paperweights. ( How much wind do you get in your office that require a paperweight?)

Thanks Gripper,

Slotted pistons are like a fairly long slow burn fuse, a bit faster if fanned often but eventually go boom even just simmering over time. Not so the oil hole kind with essentially unlimited fatigue life, like rods, something else will stop show first so if over 60s yr old think of the next owner. Don't know if ya pow wow much with seasoned vendors {while still alive] but they know stuff like this galore so ya lucky to alert ahead of time.